Deploying Email filtering for internal domains

[blason]

Hi Folks,

I have couple of internal domains hosted on Zimbra which has a same MX record which also acts as a SPAM filtering device. Like this

abc.com [192.168.1.188] and xyz.com [192.168.1.189] are Internal domain MX is mail.xyz.com [192.168.1.190] and MX for abc.com is mail,abc.com [192.168.1.190]

INTERNET ---> MX for abc.com And Xyz.com-->192.168.1.190 [smart Host Routing to different Servers]----Abc,com and Xyz.com

Now mails to and from foreign domain are getting scanned properly but what about Internal or local delivery? How can I again inject mail to my perimeter spam filtering device to get it scanned or can I add internal SPAM device and if so again how can I scan my local mail deliveries?